1. Field of the Invention
This invention relates to a multilayer piezoelectric element, and more particularly to a vibration-wave drive device.
2. Description of the Related Art
Piezoelectric material having an electrical-mechanical energy conversion function for converting electrical energy into mechanical energy is being used for various purposes. In particular, multilayer piezoelectric elements formed by laminating, integrating and sintering multiple layers of piezoelectric material are being used. A laminated layer results in large deformation distortions and large forces with a low applied voltage as compared to a piezoelectric element consisting of a single plate-shaped piezoelectric body. Also, the thickness of a laminated layer can be larger so that a small high-performance multilayer piezoelectric element can be easily manufactured.
For example, U.S. Pat. Nos. 6,046,526 and 5,770,916 dislose manufacturing techniques for a vibration-wave motor serving as a vibration-wave drive device. In particular, disclosed is a multilayer piezoelectric element serving as a multilayer electrical-mechanical energy conversion element constituting part of a vibration body of the vibration-wave motor formed in a bar shape. For uses other than the vibration-wave motor, many techniques regarding multilayer piezoelectric elements are proposed.
The multilayer piezoelectric element consists of layers of piezoelectric material (formed by a plurality of piezoelectric ceramics) and electrode layers (hereinafter, internal electrodes) provided on the surface of the respective piezoelectric layers and formed by electrode material. The piezoelectric layers and the internal electrodes are multiply laminated and then sintered. After sintering, they are polarized to have a piezoelectric property. That is, it is generally the case for a multilayer piezoelectric element that the plurality of internal electrodes are arranged over the multilayer piezoelectric element and the piezoelectric layer is a piezoelectric active part having a piezoelectric property.
FIG. 9 shows an exploded perspective view of a multilayer piezoelectric element used for a vibration body of a bar-type vibration-wave motor disclosed in U.S. Pat. No. 5,770,916.
In FIG. 9, the multilayer piezoelectric element 40 includes internal electrodes 43 provided on the surfaces of a plurality of piezoelectric layers 42. Connecting electrodes 43a (black parts in the figure) connected to the respective internal electrodes 43 and extending to outer peripheral parts of the piezoelectric layers 42 are formed on the surface of the piezoelectric layers 42. The internal electrodes 43 are arranged such that the outer periphery thereof is within the outer periphery of the piezoelectric layer 42, which is divided into four portions (AG, AG, BG, BG, A+, A−, B+, B+), and the respective internal electrodes 43 formed on the same layer are non-conductive to each other.
For every other piezoelectric layer 42, the connecting electrodes 43a are formed to be axially on the same phase positions of the multilayer piezoelectric element 40 in relation to the internal electrodes 43. The connecting electrodes 43a on the same phase position are connected by outside electrodes 44 as electrodes for continuity among the layers provided to the outer periphery of the multilayer piezoelectric element 40.
A plurality of surface electrodes 45 are provided along the periphery arround the outer periphery of the piezoelectric surface of the top layer constructing the multilayer piezoelectric element 40 and are connected to the outside electrodes 44 provided with matching the phase positions of the connecting electrodes 43a. Direct current is applied to the respective internal electrodes 43 via the surface electrodes 45, and the surface electrode 45 is polarized to provide polarized polarities for enabling the following vibration-wave motor to be driven.
FIG. 10 is a section view of the multilayer piezoelectric element 40 shown in the FIG. 9 combined to a vibration body 51 of a bar-type vibration-wave motor 50.
In FIG. 10, the multilayer piezoelectric element 40 with a penetrating hole in the center has the surface electrode 45 contacting with a flexible circuit board 52 and is arranged between hollow metallic members 53 and 54 constituting the vibration body 51. By inserting and screwing a bolt 55 into the metallic member 54 from the metallic member 53 side, the multilayer piezoelectric element 40 and the flexible circuit board 52 are placed and fixed between the metallic members 53 and 54. The flexible circuit board 52 is connected to the surface electrodes 45 connected to the outside electrodes 44 of the multilayer piezoelectric element 40 and to a drive circuit which is not shown in the figures, and high-frequency voltage for driving is applied to the multilayer piezoelectric element 40.
A rotor 58 contacting the tip of the metallic member 54 by pressing via a spring 56 and a spring support body 57 is arranged on one side of the vibration body 51 in the axial direction, and the rotation output of the vibration-wave motor 50 can be extracted by a gear 59 rotating integrally with the rotor 58.
The drive principle of the bar-type vibration-wave motor 50 is that two bending vibrations axially crossing the vibration body 51 to which the multilayer piezoelectric element 40 is assembled are generated with a time phase difference, so that the metallic member 54 moves in a swiveling manner with the tip of the metallic member 54 constituting the vibration body 51 as a drive section, and so that the rotor 58 as a contact member contacting the metallic member 54 by pressing rotates by frictional contact.
As for a linearly driven vibration-wave motor, Japanese Patent No. 3279021 and U.S. Pat. No. 5,698,930 propose using of a flat plate-shaped vibration body.
FIGS. 11A-C are schematic drawings of a linearly driven vibration-wave motor, in which FIG. 11A is a front view, FIG. 11B is a right side view, and FIG. 11C is a plan view.
In FIG. 11, two piezoelectric elements 62, 63 that concurrently generate longitudinal vibrations and bending vibrations are arranged on one side of the metallic member 61 constituting part of the vibration body. Two projections 64, 65 are formed on the other side of the metallic member 61. The two piezoelectric elements 62, 63 are adhered to an elastic body with adhesive.
High-frequency voltage A and high-frequency voltage B are respectively applied to those two piezoelectric elements 62, 63 and the compound movements of the bending vibrations and the longitudinal vibrations are synthesized, and thereby elliptic motion or circular motion can be generated to the tips of the projections 64, 65. The two piezoelectric elements 62, 63 are polarized to respectively have polarities in the same direction, and the high-frequency voltage A and the high-frequency voltage B have a time phase difference by 90 degrees.
As a result, when the tips of the projections 64, 65 are pressed and contacted to a fixing member 66, the metallic member 61 constituting part of the vibration body moves in relation to the fixing member 66. Accordingly, by pressing and contacting other members to the vibration body, a relative displacement motion is generated therebetween, and the vibration-wave motor can be driven linearly. However, in this case, the piezoelectric elements 62, 63 are single plate-shaped elements and not multilayer piezoelectric elements.
As the vibration-wave motor is compacted, the machining errors of the multilayer piezoelectric element 40 and the metallic members 53, 54 in the vibration-wave motor as shown in FIG. 10 as well as the machining errors of the metallic member 61 and the piezoelectric elements 62, 63 in the vibration-wave motor as shown in FIG. 11 would be larger compared to the entire size of the vibration-wave motor, and it would be difficult to produce in mass volume the vibration-wave motors with stable output because of accumulation of these errors.
It is also difficult to fully adhere a piezoelectric element with the interface or adhesive surface of a metallic member, and thereby, vibration damping is caused at the interface or adhesive surface and the performance of the vibration-wave motor was lowered.